Transformerless semiconductor AC switch having internal biasing means

ABSTRACT

An improved semiconductor AC switch is described having internal bias generation for the power MOSFET switches and isolated control input. Dual power MOSFETS with substrate diodes are connected in series between source and load. DC gate bias for the MOSFETS is derived from an internal power supply containing energy storage which charges from the line, typically every half cycle. The gates of the power MOSFETS are tied to the internal bias generator through a voltage divider network containing a variable resistance controlled by an optical input signal. The internal energy storage may be a capacitor or solid state battery, preferably a monolithic thick or thin film battery. No transformers or external control bias generators are required and the resulting switch is particularly simple and compact.

FIELD OF THE INVENTION

This invention relates generally to means for switching or controllingAC power, and more particularly, improved solid state switching meansemploying internal bias generation, energy storage, and control inputisolation to permit operation at high voltages.

BACKGROUND OF THE INVENTION

Historically, mechanical relays, vacuum tubes and gas discharge tubeshave been utilized to control AC power where large voltage(e.g., >100volts) are involved. More recently, semiconductor switches have beendeveloped employing bipolar transistors, field effect transistors andthyristors. Such semiconductor based switches have a number of wellknown limitations which are well known in the art. Among theselimitations is the difficulty of providing the proper DC bias to thecontrol lead of the semiconductor device. In the prior art this hasgenerally been accomplished by providing a separate power supply forbias generation or by using a transformer to provide DC isolation of thecontrol circuitry from the main AC power leads or a combination thereof.These and other prior art methods are bulky and costly and often fail toprovide adequate isolation between the control and power leads of theswitch.

Accordingly, it is an object of the present invention to provide animproved means for switching AC power using semiconductor elements, andwhich does not require external bias generation or use of a transformerfor isolating the control input.

It is a further object of the present invention to provide an improvedmeans for switching AC power using semiconductor elements and internal,transformerless, bias generation which does not require a common groundreference between power and control signals.

As used herein the words "switch" or "semiconductor switch" are intendedto refer to a device employing semiconductor elements for continuouslyvariable and/or binary (on-off) control of electrical power; the word"monolithic" is intended to refer to a structure that is formed on acommon substrate in an integrated fashion rather than being assembledfrom separate independent parts; and the words "MOSFET" or "MOSFETS" areintended to refer to insulated gate field effect devices having either Nor P type channels and various geometries.

SUMMARY OF THE INVENTION

The foregoing and other objects and advantages are realized, in apreferred embodiment, through an electrical device comprising: first andsecond power MOSFETS, wherein first power terminals of the first andsecond MOSFETS are coupled together and a second power terminal of thefirst MOSFET is coupled to a first power input means and a second powerterminal of the second MOSFET is coupled to a first power output means,and wherein the first and second MOSFETS have first and second gateelectrodes coupled together; rectifying means having a first terminalcoupled to the second power input means and a second terminal; energystorage means having a first terminal coupled to the second terminal ofthe rectifying means and a second terminal coupled to the first powerterminals of the MOSFETS; and connection means extending from the firstterminal of the energy storage means to the coupled together gateelectrodes.

In a preferred embodiment for controlling AC power, the energy storagemeans includes a DC energy storage means charged from the AC power inputand the connection means comprises a variable resistance means whoseresistance is adjusted by an optical input signal. It is desirable thatthe energy storage means be internal to the device. It is also desirablethat there be a predetermined resistance between the coupled togetherfirst power terminals and the coupled together gate electrodes. It isfurther desirable that the device of the present invention be formed ina monolithic fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit schematic of an AC semiconductor switch accordingto the prior art;

FIGS. 2-6 show circuit diagrams of an AC semiconductor switch accordingto various embodiments of the present invention;

FIG. 7 shows a simplified plan view of a monolithic implementation ofthe circuit of FIG. 6; and

FIG. 8 shows a simplified partial cut-away and front view of the deviceof FIG. 7.

DETAILED DESCRIPTION OF THE FIGURES

The circuits illustrated in FIGS. 1-6 are shown using N-channel MOSFETSas the semiconductor switches and various diodes and other componentsarranged so as to provide conduction in directions appropriate thereto.However, this is merely for ease of understanding. Those of skill in theart will understand that P-channel devices could also be used andfurther understand how to arrange the direction of conduction of thevarious diodes and other components to provide an equivalentlyfunctioning circuit based on power MOSFETS of the opposite conductivitytype.

FIG. 1 shows a circuit diagram 10 of an AC semiconductor switch of theprior art. N-channel MOSFETS 12,14 are connected in series between ACpower source 16 and load 18. The control signal to turn MOSFETS 12, 14on and off is provided by second AC source 20 through isolatingtransformer 22 and rectifying DC bias circuit 24. Bias circuit 24 iscoupled to the gates of power MOSFETS 12, 14, and consists of rectifyingdiodes 26, 28, capacitor 30 and JFET 32 with resistor 34. Because oftransformer 22, the bias provided between commonly connected sources 36and commonly connected gates 38 of MOSFETS 12, 14 need not be referencedto the same ground as the AC power source 16.

In general, the prior art circuit of FIG. 1 cannot be provided inmonolithic form since transformers suitable for use at low frequencies(e.g., <100 Megahertz and especially at standard 50-400 Hertz powerfrequencies) are not available in such form. Further, the prior artcircuit of FIG. 1 requires second AC source 20 to generate the controlvoltage for biasing the power MOSFETS into or out of conduction.

FIGS. 2-6 show circuit diagrams of an improved AC semiconductor switchaccording to various embodiments of the present invention. In firstembodiment 30 shown in FIG. 2, AC power source 32 is connected to inputterminals 34, 35 and load 36 is connected to output terminals 38, 39 ofswitch 30. Power MOSFETS 40, 42 having common source connection point 44are serially coupled between input 34 and output 38. Diodes 46, 48bridge across MOSFETS 40, 42, respectively. Diodes 46, 48 may besubstrate diodes associated with the drain regions of the power MOSFETS.In this respect it is desirable that the source regions be shorted tothe portion of the substrate which forms the substrate-drain diode.Gates 50, 52 of MOSFETS 40, 42 are tied together at common gateconnection point 56. Resistor 58 is desirably provided between commongate connection point 56 and common source connection point 44.

In order for AC switch 30 to conduct when input terminal 34 isinstantaneously positive, sufficient DC bias must be applied to gate 50to turn-on MOSFET 40. Current then flows through MOSFET 40 and diode 48to load 36. When input terminal 34 is instantaneously negative, then thefunctions of the MOSFETS and diodes are reversed and conduction is viaMOSFET 42 and diode 46. In this respect the functioning of the presentcircuit is the same as the prior art circuit of FIG. 1.

However, unlike the prior art circuit of FIG. 1, the present inventionshown in FIG. 2 does not require a separate oscillator and transformerfor generating the control bias voltage to be applied to the gates ofthe power MOSFETS. As shown in the embodiment of FIG. 2, the presentinvention employs a bias generator 60, 62 which derives its power fromthe same AC input 34, 35 which provides (through switch 30) power toload 36.

Energy storage module 60 is charged from the AC line at least during aportion of the AC waveform, typically each half-cycle. In a firstembodiment, energy storage module 60 comprises rectifying diode 64,series current limiting resistor 66, and charge storage capacitor 68.Zener diode and filter capacitor 72 are conveniently used to regulatethe voltage across capacitor 68. Terminal 65 of energy storage module 60is connected to input terminal 35 and terminal 67 is connected to commonsource connection point 44. These connections provide the AC pathway ofenergy storage module 60.

Variable resistance 62, for example, a MOSFET is provided between DCoutput 69 of energy storage module 60 and common gate connection point56. Resistances 62 and 58 provide a voltage divider. When variableresistance 62 is high (e.g., MOSFET 63 turned off), then resistor 58pulls common gate connection point 56 to the potential of common sourceconnection point 44 and MOSFETS 40, 42 are turned off. When variableresistance 62 is low (e.g., MOSFET 63 turned on) then the voltagebetween common source connection point 44 and common gate connectionpoint 56 will rise to the voltage determined by the ratio of resistance58 and resistance 62 and the voltage of DC output 69 of energy storagemodule 60.

Control module 74 comprising, in this embodiment, transistors 75, 76,resistors 77-79 and photovoltaic diode stack 80, provides the biasnecessary to turn variable resistance 62 on or off in response toexternally supplied optical signal 82 derived, for example, from LED 84.Diode stack 80 must have enough series connected diodes therein togenerate a voltage above the threshold of MOSFET 63 to fully turn it onso that, in turn, most of the voltage provided by module 60 appearsacross the gates of MOSFETS 40, 42 to turn them on. When light 82strikes diode string 80, a voltage appears at the gate of transistor 76turning it on, which then pulls the gate of transistor 75 below itsthreshold voltage, tuning off transistor 75 so that it cannot short outthe voltage from diode stack 80. Resistor 77 and transistor 75 insurethat when there is no light falling on diode stack 80, the gate oftransistor 63 does not charge up and inadvertently turn on device 63and, in turn, devices 40, 42.

It has been determined that an AC semiconductor switch capable ofswitching about 4 amps at 120 volts can be implemented according to FIG.2 where, conveniently, resistor 58 has about 3k ohms, resistors 66 and79 have about 30k ohms, resistor 78 has about 50k ohms and resistor 77has about 75k ohms. Resistors of such values occupying reasonable areasare readily formed from lightly doped or undoped polysilicon, but otherresistor materials well known in the art may also be used. While theseresistor values are convenient, other values are useful, so long as theyprovide the resistor ratios appropriate to yield the voltage divideraction already discussed. A ratio of about 10:1 in the off-state andabout 1:1 in the on-state are convenient for variable resistance means62 and fixed resistance 58. Higher currents and/or voltages can beswitched by using larger transistors 40, 42 and/or transistors withgreater voltage stand-off capability.

For 50-60 cps operation, for example, capacitors 68, 72 are convenientlyabout 1 microfarad although larger or smaller capacitors can also beused. Capacitor 72 sees the peak line voltage but capacitor 68 isprotected by resistor 66 and Zener diode 70. Capacitor 72 suppliescharge through resistor 66 when the line voltage is lower than peak oron the opposite half cycle. The size of the energy storage capacitors isdetermined by the time constant of the power MOSFET gate capacitance andresistor 58. By increasing the value of resistor 58 while maintaining anadequate resistance ratio of variable resistance means 62 and resistance58, the amount of energy storage capacitance decreases. Sufficientenergy must be stored in the combination of the gate capacitance and theenergy storage capacitance so that the gates of power MOSFETS 40, 42will remain on during the half-cycle when energy is not being suppliedto energy storage means 60 and gate connection point 56 from the line.

Zener diode 70 conveniently has a breakdown voltage of about 8 volts.Photovoltaic diode string 80 should deliver about 6 volts and 50microamps when illuminated.

Voltage regulator diodes and photovoltaic diodes are formed by providingadjacent semiconductor regions of predetermined doping using means wellknown in the art. Common semiconductor device fabrication techniques maybe employed to form capacitors 68, 72 in energy storage module 60. It isdesirable to use higher dielectric constant materials, such as forexample silicon nitride, as the capacitor dielectric in order to reducethe occupied area of the capacitors.

FIG. 3 shows an improved AC semiconductor switch according to a furtherembodiment of the present invention in which energy storage module 60 ofFIG. 2 has been replaced by energy storage module 92 comprisingrectifying diode 94, series resistance 96 and battery 98. Output points65, 67 and 69 of module 92 have the same connections to other circuitpoints as with energy storage module 60 of FIG. 2.

While battery 98 can be a conventional storage cell, it is desirablethat battery 98 be monolithic, that is, be formed on the same substrateas the remainder of switch 30. Batteries which employ solid electrolytesare well known in the art. Examples are described by A, Hooper in "SOLIDSTATE BATTERIES FOR ELECTRONICS AND MICROELECTRONICS APPLICATIONS",Electric Vehicle Developments (UK), Vol. 6, No. 3, July 1987, pages99-101, by A. Hooper and B. Tofield in "ALL SOLID STATE BATTERIES",Journal of Power Sources, Vol. 11, Jan-Feb 1984, pages 33-41, and by J.Owen in "MICRO-BATTERIES", Solid State Batteries, Proceedings of theNATO Advanced Study Institute, 1985, Pages 413-422. Solid statebatteries that can be formed as thick or thin films or a combinationthereof are described therein. Thus, by forming, for example, a thinfilm solid state battery on the same substrate as used for thetransistors, resistors and diodes, using means known in the art, amonolithic switch with integral energy storage capability is provided.

As those of skill in the art will appreciate, because MOSFETS are usedfor the power switching devices, the amount of energy that must bestored either in capacitor 68 of FIG. or battery 98 of FIG. 3 is small.Further, the energy need not be stored for a lengthy period of timesince the stored energy is frequently refreshed, typically but notessentially, every half-cycle. This energy storage refresh occurs aslong as the switch is connected to the line no matter whether the switchis turned "off" or "on".

FIG. 4 shows a further embodiment of switch 30 in which control modules74 of FIGS. 2-3 has been simplified and replaced by control module 104.Control module 104 comprises diode string 80 and resistor 99. Diodestring 80 is, as before, energized by light 82. Resistor 99 allows thecharge developed on diode string 80 and stored, for example, on the gateof MOSFET 63 to dissipate when light 82 is removed. This allows MOSFET63 to return to its high impedance state, turning off power MOSFETS 40,42. The arrangement of module 104 in FIG. 4 has the advantage ofrequiring fewer components than the arrangement of module 74 in FIGS.2-3, but the disadvantage of requiring more drive from diode stack 80(e.g., larger diodes or more light) to provide current through resistor99 and keep the gate of transistor 63 well above threshold so that itson-resistance stays low.. Those of skill in the art will understand thatthe arrangement of FIG. 4 may utilize energy storage means 92, as shown,or energy storage means 60 of FIG. 2.

FIG. 5 shows a further embodiment of the present invention whereinenergy storage means 60 of FIG. 2 or 92 of FIG. 4 is replaced by energystorage means 102 comprising rectifier diode 94, voltage regulatordiodes 100 and battery 98. Voltage regulator diodes 100 provide apredetermined maximum voltage drop. One or more voltage regulator diodesmay be used depending upon the magnitude of voltage desired, whichvaries with the desired line operating voltage, as those of skill in theart will appreciate based on the description herein. Output connections65, 67, and 69 of means 102 are connected as before and the circuit ofFIG. 5 functions in the manner previously described, taking into accountthe internal differences in energy storage means 102.

FIG. 6 shows a schematic of a further embodiment of the invention whichis particularly simple and which has the further advantage of ease ofconstruction. Variable resistance means 62 and control means 74 or 104have been combined and replaced by photosensitive variable resistancemeans 114. Those of skill in the art will understand that the circuit ofFIG. 6 can utilize energy storage means 102, as shown, or either ofpreviously described energy storage means 60 or 92, or an equivalent.Light 82 shining on variable resistance means 114 causes the value ofresistor 109 to drop from a high (dark) value to a lower (illuminated)value. The DC voltage provided by energy storage means 102 (or 60 or 92)at connection 69 divides across the voltage divider formed by resistors109 and 58. By properly selecting the values of resistor 58 incomparison to the dark and light resistance values of photosensitivevariable resistor 109, the source-gate voltage at common gate connectionpoint 56 is made to swing from below threshold to significantly abovethreshold (but less than breakdown) when photosensitive resistor 109 isilluminated. Those of skill in the art will understand how to choose thevarious resistance values based on the description given herein. Themaximum source-gate voltage appearing at common gate connection point 56is limited by the output voltage provided by energy storage means 60,92, or 102. This is less than the gate breakdown voltage of powerMOSFETS 40, 42, regulated either by Zener 70 for energy storage means 60or by the voltage of battery 98. In general, battery 98 acts both as anenergy storage means and a voltage limiter. If additional voltagelimiting action is desired in the arrangements of FIGS. 4-6, then anadditional voltage regulator may be placed across battery 98.

Photosensitive variable resistor 109 is conveniently provided by meansof a region of semiconductor material either in the semiconductorsubstrate used for forming the power MOSFETS and/or the other devices,or by means of a layer of semiconductor material, e.g., polycrystallinesemiconductor, deposited on a dielectric layer on the substrate surface.Polysilicon is a particularly convenient material, since its resistivitycan be adjusted across broad ranges by doping, it is known to be highlyphotosensitive, and its fabrication as a photosensitive variableresistor may be accomplished by the same techniques, well known in theart, used for forming power MOSFET 40, 42 and other parts of switch 30.Photo-resistor 109 can also be a photosensitive FET.

FIG. 7 is a plan view in highly simplified form of monolithic AC switch118 of the present invention showing how the circuits illustrated inFIGS. 2-6, and especially the circuit of FIG. 6, may be realized. As anaid to understanding and as is conventional in the art, the variousoverlapping regions and layers of the device of FIG. 7 have been shownas if transparent so that their relative position may be easilydetermined. Where possible, the various regions in FIGS. 7-8 have beenidentified with the same reference numbers used in FIG. 6 so that thecorrespondence between the circuit diagram of FIG. 6 and the physicalrealization of FIGS. 7-8 may be readily understood. FIG. 8 is asimplified partial cut-away and front view of the device of FIG. 7. Inthe cut-away portions of FIG. 8, the semiconductor substrate is shownclear, regions that are conveniently fabricated of polycrystallinesemiconductor, e.g., polysilicon, are shown stippled, dielectric regionshave widely spaced hatching, and metal or other conductor regions havenarrowly spaced hatching.

AC switch 118 comprises power MOSFETS 40, 42. While MOSFETS 40, 42 areillustrates as conventional lateral MOSFETS formed in semiconductorsubstrate 119, those of skill in the art will appreciate that DMOS orTMOS types of devices with top contacts could also be used. MOSFET 40has drain 120, source 121 and gate 50. Input connection 34 is coupled todrain 120. MOSFET 42 has drain 122, source 123 and gate 52. Outputconnection 38 is coupled to drain 122. Isolation region 130 separatesdrains 120, 122. Sources 121, 123 are coupled by conductive connection44 which also attaches to end 58A of resistor 58 and to terminal 132 ofbattery 98.

In the embodiment illustrated in FIGS. 7-8, battery 98 lies above powerMOSFETS 40, 42 and is indicated in FIG. 7 by the heavy outline. Forconvenience in viewing the underlying components, battery 98 is taken astransparent in FIG. 7. Battery 98 desirably comprises a stack ofpositive and negative conductive plates or layers 98A, 98B separated bysolid electrolyte 98C. As those of skill in the art will understandbased on the description herein, the number of series connected cells(i.e., - layers) will depend upon the individual cell voltage and thethreshold voltage of devices 40, 42. Battery 98 is formed by means wellknown in the art and is desirably integral with switch 118. Beforeforming battery 98 it is desirable to provide a dielectric planarizationlayer over devices 40, 42 to give a substantially smooth surface forreceiving battery 98 and to act as a passivation layer to protects theunderlying components from the materials used to form battery 98.Silicon nitride, silicon dioxide, doped glasses and mixtures thereof, ororganic polymers such as for examples polyimides, are examples ofsuitable planarizing and passivating materials. Such materials areformed by means well known in the art.

Gates 50, 52 are commonly connected, for example at 51 across isolationregion 130, and coupled to end 58B of resistor 58 and to end 109A ofphoto-resistor 109. End 109B of photoresistor 109 is coupled to terminal134 of battery 98 and to connection point 136 of Zener diodes 100 whichare in turn connected to rectifying diode 94. A suitable arrangement ofP and N regions formed in polysilicon is illustrated in the cut-awayview of FIG. 8. Diode 94 is coupled to terminal 65 where wire bond 138for example, see FIG. 8, is used to make external connection to thereference terminal of switch 118 For simplicity, wire bond 138 isomitted from FIG. 7.

Dashed lines 140, 142 which surround respectively optically sensitivedevice 109 and charging circuit 94, 100 are intended to indicate thatthe structures therein in FIGS. 7-8 may be replaced by other embodimentsof the photosensitive and charging means (including resistance means62), as for example those shown in FIGS. 2-5. The battery likewise maybe replaced by the energy storage means illustrated in FIG. 2.

Having thus described the invention it will be apparent to those ofskill in the art that a particularly compact and improved ACsemiconductor switch is provided. A desirable result of the inventedarrangement is that isolation which will withstand several kilovoltspotential difference between control and power circuits is provided andno common ground or reference is need between control and powercircuits. They are optically isolated. A further advantage of thepresent invention is that no transformer is required in the controlcircuit and no separate external bias generator need be employed. Hence,the invented circuit, structure and method is particularly simple,economical and suited to monolithic implementation.

Those of skill in the art will also appreciate that variations may bemade on the present invention without departing from the scope thereof.For example and not intending any limitation, while the embodiments havebeen described as using various combinations of MOSFETS and P and Nregions, those of skill in the art will appreciate that P and N may bereversed and that other types of devices may also be used. Further,while the implementation has been described as being desirably made on asemiconductor substrate, other substrates suitable for device formationmay also be used. Accordingly, it is intended to include such variationsin the claims that follow.

I claim:
 1. A transformerless AC semiconductor switch for coupling ACpower to an AC load, comprising:first and second power input means forreceiving power from an AC source; first and second power output meansfor coupling AC power to a load; first and second series connectedMOSFETS having connected together first power terminals and second powerterminals coupled between the first power input means and the firstpower output means, wherein the MOSFETS have connected together gateelectrodes; and internal voltage bias means coupled between theconnected together first power terminals and the connected together gateelectrodes of the MOSFETS for providing a gate voltage larger than thethreshold voltage of at least one of the power MOSFETS, wherein theinternal voltage bias means generates the bias from the first and secondpower input means without use of a transformer.
 2. The switch of claim 1further comprising first and second diode means between the powerterminals of the first and second MOSFETS, respectively, and arranged inserial opposed fashion between the power input and output means.
 3. Theswitch of claim 1 wherein the internal voltage bias means comprises afirst rectifying means having a first terminal coupled to the secondpower input means and a second terminal coupled to the connectedtogether first power terminals through at least one capacitor andcoupled to the connected together gate electrodes through a currentlimiting means.
 4. The switch of claim 3 further comprising a resistorand a voltage reference means, wherein the resistor has a first terminalconnected to a first terminal of the at least one capacitor and to thesecond terminal of the rectifying means and a second terminal connectedto a first terminal of the voltage reference means and wherein thevoltage reference means has a second terminal coupled to a secondterminal of the at least one capacitor.
 5. The switch of claim 4 furthercomprising a controllable variable resistance means having a first powerterminal coupled to the second terminal of the resistor and a secondpower terminal coupled to the connected together gate electrodes.
 6. Theswitch of claim 5 further comprising energy storage means coupled acrossthe voltage reference means.
 7. The switch of claim 5 wherein theresistance of the controllable variable resistance means varies inresponse to an optical input signal.
 8. The switch of claim 7 whereinthe controllable variable resistance means comprises a photovoltaicdevice.
 9. A transformerless AC semiconductor switch for coupling ACpower to an AC load, and having internal biasing means, comprising:firstand second power input means for accepting AC power; first power outputmeans for coupling AC power to a load; first and second series opposedconnected MOSFETS having coupled together first power terminals andsecond power terminals coupled, respectively, to the first power inputand first power output means, wherein the MOSFETS have coupled togethergate electrodes; and internal voltage bias means coupled between thecoupled together first power terminals and the coupled together gateelectrodes of the MOSFETS for providing a gate voltage larger than thethreshold voltage of at least one of the power MOSFETS, wherein thevoltage bias means comprises an integral energy storage means coupled tothe first and second power input means for receiving energy therefromduring at least some half cycles of the AC power.
 10. The switch ofclaim 9 wherein the integral energy storage means comprises a capacitor.11. The switch of claim 9 wherein the integral energy storage meanscomprises a diode whose input is coupled to the second power inputmeans, a resistor and a capacitor having first terminals coupled to thediode output, and a voltage reference means coupled across secondterminals of the resistor and capacitor.
 12. The switch of claim 11further comprising means for coupling the juncture of the capacitor andvoltage reference means to the coupled together first power terminals,and means for coupling the juncture of the resistor and the voltagereference means to the coupled together gate electrodes.
 13. The switchof claim 12 further comprising resistance means coupled between thecoupled together first power terminals and the coupled together gateelectrodes.
 14. The switch of claim 9 wherein the integral energystorage means comprises a solid state battery.
 15. The switch of claim14 wherein the integral energy storage means is monolithic with theswitch.
 16. The switch of claim 14 wherein the energy storage meanscomprises a diode coupled between the second power input means and afirst terminal of the battery and wherein a second terminal of thebattery is coupled to the coupled together first power terminals.
 17. Atransformerless AC semiconductor switch for coupling AC power to an ACload, and having internal energy storage means, comprising:first andsecond power input means for accepting AC power; first and second poweroutput means for coupling AC power to a load; first and second MOSFETShaving coupled together first power terminals and having a second powerterminal of the first MOSFET coupled to the first power input means andhaving a second power terminal of the second MOSFET coupled to the firstpower output means and having coupled together gate electrodes;rectifying means having a first terminal coupled to the second powerinput means and a second terminal; energy storage means having a firstterminal coupled to the second terminal of the rectifying means and asecond terminal coupled to the coupled together first power terminals ofthe MOSFETS; and connection means extending from the first terminal ofthe energy storage means to the coupled together gate electrodes. 18.The switch of claim 17 wherein the connection means comprises a variableresistance means whose resistance is adjusted by an optical inputsignal.
 19. The switch of claim 18 further comprising resistance meansextending between the coupled together first power terminals and thecoupled together gate electrodes.
 20. The switch of claim 17 wherein therectifying means comprises a rectifying diode in series with a least oneopposed voltage reference diode.